1. Field of the Invention
This invention generally relates to thermosetting polyurethane compositions and specifically relates to cross-linkable compositions comprising a resin having hydroxy, carboxylic acid, amide or carbamate functional groups, and a mixture of monomeric and oligomeric polyfunctional substituted urethane compounds.
2. Description of the Prior Art
In the broad practice of the coatings art, it is known to formulate coatings containing resins with free hydroxyl groups which are blended with melamine resins under acidic conditions to provide film-forming lacquers. The dry film coatings produced by such compositions are tough and exhibit high resistance to degradation caused by air oxidation and light exposure.
U.S. Pat. No. 3,557,043 discloses an improved coating composition based on polyacrylate resins with hydroxyl functionality and containing N-alkoxy-methylurethanes, wherein from about 0 to about 70% by weight of the urethane component may be replaced by a melamine formaldehyde and/or urea formaldehyde resin. Such urethane-based compositions are disclosed as having high elasticity, impact strength and solvent resistance relative to the lacquer compositions mentioned above.
The N-alkoxy-methylurethanes employed in the compositions discussed immediately above are disclosed as being obtainable as a reaction product of a polyhydroxyl compound and an alkoxymethyl isocyanate of the formula ROCH.sub.2 --NCO, wherein R is an alkyl or alkylene radical having one to 12 carbon atoms. Unfortunately, however, such alkoxymethyl isocyanates are highly toxic in character and are at least suspected carcinogens. Further although the N-alkoxy-methylurethanes employed in the prior art coating compositions is polyfunctional in character, such urethanes are monomeric in character and thus do not provide the cross-linkability associated with oligomeric or polymeric polyfunctional compounds.
Accordingly, it is an object of the present invention to provide a coating composition containing a cross-linkable polymer and polyfunctional substituted urethane compounds, wherein the latter may be prepared from safe, easily handled, non-toxic starting materials, with the resultant coating composition being curable at elevated temperature to provide a highly cross-linked coating having high solvent resistance, and otherwise good dry film properties, as well as high resistance to weathering, including resistance to light and oxidative degradation.